1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copying machine, a printer or the like, and more particularly, to an image forming apparatus in which an intermediate guide means for a flexible, sheet-like transfer medium is disposed at a certain position in a feed passage between a transfer section and a fixing section.
2. Description of the Prior Art
Conventional types of image forming apparatus have been arranged in the following manner. A transferable toner image is formed on the surface of an image carrier which is rotating. A flexible, sheet-like transfer medium which travels in synchronism with the image carrier is brought in close proximity to or in contact with the toner image formed on the surface of the image carrier. After the toner has been transferred to the transfer medium, the medium is separated from the image carrier and is conveyed to a fixing section by given feed means.
Referring to FIG. 7 showing one example of such a conventional type of image forming apparatus, a transfer medium P, such as copying paper, is supplied from a paper feed section (not shown) and is synchronized by a pair of frictional rollers 12 and 13. The thus-synchronized transfer medium P reaches a transfer section through guide means 2 and 2'. In the transfer section, the toner image formed on an image carrier drum 1 is transferred to the transfer medium P by the electric discharge of a transfer charger 3. After completion of the image transfer, the medium P is fed toward a pair of fixing rollers 5a and 5b by a feed belt 4.
Hitherto, various proposals have been made with respect to a fixing device for fixing a toner image on a transfer medium. In particular, two methods have widely been put into practical use. One is a heat-pressure method in which a pair of rollers are pressed against each other and at least one roller of the pair is heated so as to melt toner, thereby fixing the melted toner to the transfer medium. The other method is a pressure-fixing method in which toner is fixed to a transfer medium under pressure alone, without using any heating means.
In the case of such fixing methods using the above-described roller pair, when the leading end of the transfer medium is inserted into the nip between the fixing roller pair, the load applied to the fixing roller pair is abruptly increased and rotational speed of the fixing roller pair is instantaneously decreased. The force created by the instantaneous decrease in the rotational speed acts to push back the transfer medium and the synchronism between the transfer medium and the image carrier drum becomes unbalanced in the transfer section, so that image formation is adversely affected.
In order to cope with such a problem, one proposal has been made in which intermediate guide means constituted by an upper-surface roller 6a and an underside roller 10a, as shown in FIG. 7, is disposed at a certain position between the transfer section and the fixing section, and the feed belt 4 is nipped between the rollers 6a and 10a. According to this construction, when a force acting to push back the transfer medium is generated in the nip between the pair of fixing rollers 5a and 5b, the force can be absorbed by the slack which is formed in the portion of the transfer medium between the pair of fixing rollers 5a and 5b and the upper-surface and underside rollers 6a and 10a.
Specifically, in the pressure-fixing method, the rollers which are pressed against each other are disposed in such a manner that the axes cross each other at a slight angle in order that pressure is uniformly applied to the transfer medium along the entire length of the respective fixing rollers. FIG. 8 shows the state wherein the fixing rollers 5a and 5b intersect each other at an angle of .alpha.. In the case of a fixing section constructed in this fashion, the transfer medium P is fed at an angle of .alpha./2 degrees with respect to each of the fixing rollers 5a and 5b. However, since the arrangement shown in FIG. 8 is such that the two rollers 5a and 5b are respectively shifted .alpha./2 degrees from the normal with respect to the direction of travel of the transfer medium P, the transfer medium P is inserted substantially perpendicularly to the nip line where the two rollers are maintained in contact with each other, thereby causing a large impact. Accordingly, the normal arrangement, as shown in FIG. 9, is such that the roller pair is shifted at an angle of .beta. as a whole, thus allowing the transfer medium P to be inserted slantwise with respect to the nip line. In the fixing section which is constructed in this manner, the transfer medium P travels in the direction of an arrow B before it is inserted into the nip between the two rollers. However, after the medium P has passed through the nip, it travels in the direction of an arrow C and deviates from the original direction of travel as indicated by the arrow B. In consequence, a wrinkle 9, as shown in FIG. 9, is produced as a result of the unnatural force acting on the transfer medium P. If, as shown in FIG. 10, the upper-surface rollers 6a and 6b are disposed parallel with each other and with the direction of travel of the transfer medium P, the wrinkle 9 passes in the reverse direction through the space between the upper-surface rollers 6a and 6b, and one end of the wrinkle 9 reaches the transfer section, with the result that image formation is adversely affected.
In order to prevent the toner on the transfer medium from sticking to the upper-surface rollers 6a and 6b, one method, as shown in FIG. 11, has been proposed in which an electric field is formed between the upper-surface rollers 6a and 6b and underside rollers 11a and 11b by virtue of the application of a bias voltage by means of an electrical power source 7. This method also involves disadvantage in that, if the wrinkle 9 invades the space between the upper-surface rollers 6a and 6b, the positive charge Q of the toner clinging to the wrinkle 9, for example, leaks to one side 6b' of the upper-surface roller 6b, and this phenomenon is accompanied by the corresponding toner sticking to the side 6b'.